Coronavirus disease 2019 (COVID-19) is an infectious disease caused by the SARS-CoV-2 virus. First identified in late 2019, it rapidly escalated into a global pandemic, altering daily life across the world. The virus causes a respiratory illness that ranges from mild to severe, and most individuals recover without specialized medical treatment. However, certain populations, including older adults and those with existing health conditions like cardiovascular disease or cancer, face a higher likelihood of developing a serious illness.
The SARS-CoV-2 Virus and Transmission
SARS-CoV-2 belongs to a large family of viruses known as coronaviruses, and its name is an abbreviation for Severe Acute Respiratory Syndrome Coronavirus 2. A defining feature is its surface spike proteins, which enable it to attach to and enter human cells by binding to a receptor called angiotensin-converting enzyme 2 (ACE2).
The primary way SARS-CoV-2 spreads is through the air. When an infected individual coughs, sneezes, speaks, or breathes, they release respiratory particles containing the virus. These particles exist in a range of sizes, from larger droplets to smaller aerosols that can remain suspended in the air.
Inhalation of these virus-containing particles is the most common route of infection. The virus can also be transmitted if these particles land on a person’s eyes, nose, or mouth, though touching a contaminated surface is a less significant mode of spread.
Clinical Presentation and Symptoms
Once a person is exposed to SARS-CoV-2, there is an incubation period before symptoms may appear, typically ranging from two to 14 days. An infected individual can be contagious for up to two days before feeling sick, and some may have no symptoms at all (asymptomatic).
The most frequently reported symptoms of an active infection include:
- Fever, cough, and fatigue
- A new loss of taste or smell
- Sore throat and headache
- Body aches
- Congestion or a runny nose
While many cases feel similar to a common cold or the flu, the disease can progress. In more severe instances, COVID-19 can lead to serious complications like pneumonia, acute respiratory distress syndrome (ARDS), blood clots, and kidney failure.
Prevention and Medical Countermeasures
A foundational strategy for reducing the spread of SARS-CoV-2 involves non-pharmaceutical interventions. This includes wearing well-fitted masks in crowded settings, maintaining physical distance, and practicing regular hand hygiene. Improving ventilation by opening windows or using air purifiers also helps disperse virus particles indoors.
Vaccination is a primary tool for preventing severe disease, hospitalization, and death from COVID-19. The most widely used vaccines, including mRNA and viral vector platforms, work by providing cells with instructions to make a harmless piece of the spike protein. This process triggers an immune response that prepares the body to fight off the actual SARS-CoV-2 virus if exposed.
For individuals who become infected, medical treatments are available. Antiviral medications work by stopping the virus from replicating and are often prescribed for outpatients at higher risk to lessen the severity of the illness. For patients hospitalized with severe disease, treatments may include corticosteroids to reduce inflammation and other supportive care to manage complications.
Viral Evolution and Variants
Like all viruses, SARS-CoV-2 constantly changes through a process of mutation. As the virus replicates, small errors can occur in its genetic code, leading to new versions, or variants. While most mutations are insignificant, some can alter the virus’s characteristics, such as how easily it spreads or the severity of the disease it causes.
Health organizations classify these mutations to monitor their potential impact. Variants that show evidence of increased transmissibility or cause more severe disease may be designated as “variants of concern.” This helps public health officials track their spread.
The Delta variant, for example, was known for its increased transmissibility compared to earlier strains. Later, the Omicron variant and its sublineages demonstrated an even greater ability to spread rapidly and partially evade immunity from vaccination or prior infection, though it often resulted in less severe illness on average. This continuous evolution necessitates ongoing surveillance.
Post-COVID Conditions
A significant number of individuals who have had COVID-19 experience long-term health problems, a condition often referred to as “Long COVID” or post-COVID conditions. This can occur in anyone who has been infected, regardless of whether their initial illness was severe or mild. These post-illness effects can persist for weeks, months, or even longer after the acute phase of the infection has ended.
The range of reported symptoms is broad and can affect multiple organ systems. Common persistent symptoms include chronic fatigue, shortness of breath, and cognitive dysfunction, often described as “brain fog.” Other reported issues include persistent cough, muscle and joint pain, heart palpitations, and problems with smell or taste.
The underlying biological causes of post-COVID conditions are not yet fully understood and are an area of intensive scientific research. Investigators are exploring several possibilities, including the persistence of the virus in certain body tissues, chronic inflammation, and autoimmune responses triggered by the initial infection.